Late Pleistocene Glaciers and the Climate of Western Montana, U.S.A.

Abstract

Western Montana is ideally situated to have both its climate affected by the presence of Pleistocene ice sheets and those effects be apparent in its distribution of mountain glaciers. Paleoequilibrium line altitudes (paleoELAs) determined through a weighted average of cirque floor elevations, highest lateral moraines, and interpreted glacial extents define a complex pattern which reflects sources of moisture, directions of airflow, and sites of local convergence and divergence of airmasses.

The regional trend of paleoELAs is parallel to that of present glacier ELAs but lies about 450 m lower. The parallelism suggests that the regional controls on moisture availability during late Pleistocene time were similar to those at present. Numerical reconstructions of precipitation required to maintain glaciers assuming a uniform regional 10°C summer temperature depression indicate a Pleistocene decrease in precipitation, relative to present, of about 25 cm H₂O. Decreases were greater (more than 50 cm H₂O) over the high mountains of southwest Montana and snowfall may have increased (by more than 50 cm H₂O) in west-central Montana. Glacial Lakes Missoula and Great Falls may have served as significant local moisture sources during periods of maximum glaciation. Windflow patterns were similar to those at present, except that convergence of prevailing westerlies and katabatic winds from the Laurentide and Cordilleran ice sheets apparently generated local forceful uplift and increased precipitation.